This study will determine the variability of the frequency and severity of obstructive apnea in children as measured by two PSGs performed no more than 4 weeks apart. The variability measured will be a composite of the physiologic variability within individuals, the PSG signal- quality variability, and the PSG scoring variability. The scoring variability will be defined further by a substudy involving replicate scoring of a selection of PSGs. Data from this study will help to characterize better respiratory disturbance during sleep in children, and are critical to the interpretation of future studies examining the natural history of OSAS and the efficacy of treatment.